LUBBOCK, Texas — Cattle will not be allowed to leave the Texas ranch that produced the nation’s first homegrown case of mad cow disease, and government officials will work to find animals related to the sick cow, authorities said Thursday.

None of those “animals of interest” have yet been identified. If found, the cattle will be killed and tested, Texas animal health officials said.

The 12-year-old beef cow was born, raised and used for breeding at the same ranch and had never left the property, authorities said. They would not identify the ranch or the size of the herd.

Agriculture officials announced Wednesday that the cow had tested positive last fall for the brain-wasting disease. The cow, a “downer” that could not walk, never made it into the nation’s human food supply. The animal was sent to a pet food plant in Waco, Texas.

It was the first time the disease has been confirmed in a U.S.-born cow. The other U.S. case was in a dairy cow imported from Canada.

The state Animal Health Commission put a hold on the ranch’s cattle earlier this month when tests indicated a mad cow case among the herd.

Officials have said the infection most likely started with contaminated feed eaten before August 1997, when the United States and Canada began banning cow parts in cattle feed. The cow was born about four years before the feed ban was implemented.

Officials also are trying to identify herd mates born within one year of the infected cow’s birth, as well as any offspring born within the past two years and other related cattle.

USDA Chief Veterinarian John Clifford said it is “highly unusual” to find the disease in more than one animal in a herd or in an affected animal’s offspring.

The animal arrived dead in November at Champion Pet Food. An initial screening indicated the presence of mad cow, but more sophisticated follow-up tests were negative, and samples were sent to a British lab, which found that the animal had the disease. The carcass was later incinerated.

Meanwhile, the Food and Drug Administration will trace the feed history of animals born on the ranch around the same time as the infected cow, including any animals no longer on the farm.

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Investigators will also examine compliance records for plants that may have processed meat and bone meal from relatives or herd mates of the sick animal to see whether the companies complied with the feed ban regulations.

The Agriculture Department began monitoring cattle more aggressively after the first U.S. case of mad cow disease was discovered in December 2003 in Washington state. More than 400,000 cattle have been tested since June 2004.

Texas is the leading cattle state in the nation with 13.8 million head or 15 percent of the U.S. cattle inventory.

Mad cow disease, medically known as bovine spongiform encephalopathy, or BSE, is a brain-wasting illness that infects cattle. It is believed to be spread when a cow eats meal that contains spinal or brain tissue of an animal infected with BSE. Humans can get a related illness, variant Creutzfeldt-Jakob disease, if they eat infected tissue.